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I have soft water in my tank, 3 deg., and I add co2 via a controller setup. During the day the ph goes to about 6.2-6.3. The thing that seems odd to me is that at night, since my co2 turns off at lights out,the ph doesn't really rise at all, only to 6.4. I have calibrated my Mil. ph controller recently and it seemed o.k. I thought at night the co2 should outgas from the water which it doesn't seem to be doing. The fish are all fine, no gasping at the surface in the morning or any other signs of ill-effects, but I'm just wondering what the story is . Thanks,
John
 

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My tanks don't vary by more than 0.3pH units max during day/night. I haven't really thought about why, but I assume, since I have a glass top on the tank and the filter flow doesn't create any disturbance, the fluctuations are minimal.
 

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I noticed my ph swing is variable depending on the Bio-Load in the tank. The more plants and fish I have the smaller the ph swing
the less plants and fish then there is more of a ph swing.

So I figure that the more plants and fish you have the more CO2 is being expelled by them. So while you may be outgassing the CO2 that was injected during the day some of that is being replaced by the Bio load. The greater the bio load the greater the CO2 being expelled thus the smaller ph swing.

This is just a guess, I could be way off.
 
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